1. Field of the Invention
The present invention relates to improvement of a normally open hydraulic control valve comprising: a linear solenoid unit; a spool driven by an output force of the linear solenoid unit to move forward; a valve body in which the spool is slidably fitted; and a return spring for biasing the spool in a retreating direction, the valve body including: a supply port leading to a hydraulic pressure source; an output port leading to a hydraulic pressure operating unit; a drain port opened into an oil tank; a reaction force oil chamber for introducing hydraulic pressure, which presses the spool in a direction against a biasing force of the return spring, from the output port; and a damper oil chamber to which one end surface of the spool is faced and which suppresses vibration of the spool, the spool being constructed to provide blockage between the output port and the drain port at a retreat limit of the spool to provide communication between the supply port and the output port, and to provide blockage between the supply port and the output port by advance of the spool to provide communication between the output port and the drain port.
2. Description of the Related Art
An example of such a conventional normally open hydraulic control valve is shown in FIG. 8. In this example, at least the tip end of the valve body, which is at the opposite side from the linear solenoid unit, is submerged in oil of the oil tank, and the damper oil chamber communicating with the oil via the orifice is provided in the valve body, so that the damper oil chamber is always filled with oil. When the spool vibrates, the vibration of the spool is suppressed by the throttle resistance of the orifice against the oil.
The conventional normally open hydraulic control valve as described above has a restriction on placement, that is, the damper oil chamber has to be submerged in the oil of the oil tank to fill the damper oil chamber with oil. In addition, the difference in level between the oil in the oil tank and the damper oil chamber is comparatively small, and therefore supply of the oil from the oil tank to the damper oil chamber by utilizing the difference lacks swiftness, thus causing a fear that a delay in operation of the damper oil chamber occurs at the early stage of the operation of the normally open hydraulic control valve.